Radio receiver



Sept. 8, 1936. G. l.. BEERs RADIO RECEIVER Filed oct. 28,/ 195s PatentedSept. 8, 1936 UITED STATES PATE? @FFQE RAnlo RECEIVER George L. Beers,Collingswood, N. J., assignor to Radio Corporation of America, acorporation of Delaware My invention relates to radio receivers, andmore particularly is an improvement over the system disclosed in myprior application, Patent #2,010,131, issued August 6, 1935, andassigned tc Radio Corporation of America.

In the application referred to, thesystem disclosed comprises aplurality of thermionie de vices for amplifying and detecting purposes,a plurality of thermionic devices the function of which is to modify thecharacteristics of coupling networks interposed between the rstmentioned thermionic devices, and a thermionic device the function ofwhich is to supply controlling potentials to certain of thefirst-mentioned thermionic devices proportionally to the carrieramplitude of an incoming signal. By reason of the inter-connection ofthe various thermionic devices in the system referred to, automaticvolume control or automatic control of the selectivity and fidelity ofthe system is obtained, at will.

As pointed out in the aforementioned application, in the design of radioreceivers, there are two highly desirable characteristics that, in asense, are mutually exclusive-that is, a receiver must be sufcientlyselective to differentiL ate between incoming signals under maximum andminimum sensitivity conditions and, at the same time, reasonably highfidelity and freedom from distortion are desirable at all times.

Heretofore, fair delity has been attained through the use of inter-tubecoupling circuits having band-pass characteristics. It has beendifficult, however, to so design receivers, especially those providedwith automatic volume control means, which will exhibit a high degree offidelity, as Well as reasonable selectivity, when receiving signals fromlocal stations and still be sufficiently selective to receive weaksignals from distant stations without an unpleasant amount of backgroundnoise.

It is, accordingly, an object of my invention to provide a radioreceiver wherein high fidelity may automatically be had during thereception of strong signals, without sacrificing selectivity whenreceiving weak signals.

Another object of my invention is to provide a radio receiver whereinsmoothly continuous selectivity control may be manually accomplishedduring the reception of signals from strong local stations.

Another object of my invention is to yprovide, in a radio receiver,means for lautomatically increasing the selectivity during the receptionof weak signals.

Another object of my invention is to provide a radio receiver whereinthe functions of amplification and selectivity control are performed bythe same thermionic devices.

Another object of my invention is to provide 5 means whereby thel numberof thermionic devices in a system of the type described may be reducedto minimum.

Another object of my invention is to provide a radio receiver that, inaddition to the features l0 enumerated above, shall have automaticvolume or gain control.

A still further and more specific object of my invention is to provide,in a system of the type referred to, a single thermionic device for thel5 purpose of correlating automatic volume control and automaticfidelity control to incoming signal strength and a single manuallyoperated element or control device through the manipulation of whicheither of the aforementioned 2()A functions may be made available to thedesired degree.

The foregoing objects and other objects ancillary thereto, I prefer toaccomplish in part by utilizing in a radio receiver a plurality ofscreen 25 grid pentode-triode tubes of the type commercially known as6FT. In each tube, the pentode portion thereof is independent of thetriode portion, although it utilizes the same cathode, and it isemployed for amplification and other well known purposes, while thetriode portion is uti lized for automatic control of the selectivity andi delity of the receiver in response to variations in field strength.Specifically, such automatic control is had through circuit connectionswhereby the space current path in each triode portion is effectively inshunt to an intertube coupling device, and the bias potentials appliedto the grids of the said triode. portions are so automaticallycontrolled in response to incoming radio signals that the triodeplate-impedance thereof governs the selectivity, delity, and gain of thesystem.

Furthermore, to provide for the foregoing functions, and'to attain, atwill, automatic volume 45 control coupled with high selectivity, Iutilize a single 'thermionic device on the input circuit of whichpotentials corresponding to the amplitude of incoming signals areimpressed and in the output circuit of which is disposed a resistornetwork including a manually controllable potentiometer whereby thepotentials developed therein may be utilized for the desired functions.

The novel features that I consider characteristie of my invention areset forth with particuradio receiver including an alternative embodimentof my invention.

In both of the figures of the drawing, equivalent circuit components andconnections are similarly designated.

Although my invention is susceptible of application to radio receiversof many different types, I prefer to illustrate it, referring to thedrawing, as applied to a superheterodyne receiver including a radiofrequency amplifying tube I, hereinafter called the radio frequencytube, a first detector tube 3, a plurality of intermediate frequencyamplifier tubes 5 and 1, later referred to as first and secondintermediate frequency tubes and a second detector tube 9. Each of thesetubes I, 3, 5, and 1 is of the screen grid pentode-triode type, thepentode portion being constituted by al cathode I3, a control grid I5, ascreen grid I1, a suppressor grid I 9, and an anode 2|, while the triodeportion includes the cathode |3, a control grid 23, and an anode 25. Thesecond detector tube 9 is preferably of the diode type having a cathode21 and an anode 29 between which is connected an output resistor 3|.

The screen grids of the tubes 3, 5, and 1 are supplied with positivepotential at the requisite voltage over a common conductor 33.

In order to simplify the drawing, the heaters of the several tubes havebeen omitted and an oscillator 35 is merely indicated diagrammatically.

Any convenient source of plate and bias potentials may be employed and,since substantially all modern receivers are adapted to be energizedfrom house lighting circuits, this source is exemplified in the drawingby a rectier 31 having an output circuit including a bleeder resistor39.

The pentode portions of the radio frequency tube I and the rst detectortube 3 are impedance coupled, the coupling network including a tunedcircuit 4| connected directly to both anodes of the radio frequency tubeI and through a coupling condenser 53 to the pentode control grid of thefirst detector tube 3. The pentode portions of the first detector tube 3and the first intermediate frequency tube 5 are inter-connected througha tuned circuit 45 and a coupling con-- denser 41. Y

'Ihe pentode portions of the first and second intermediate frequencytubes 5 and 1 are intercoupled through the medium of a radio frequencytransformer 49, the primary and secondary windings of which are tunedand the coupling between them so adjusted that the transformerefficiently passes a band of frequencies l k. c. above and 10 k. c.below the chosen intermediate frequency. 'Ihe pentode portion of thesecond intermediate frequency tube 1 and the second detector tube 9 areinter-coupled through a band-pass radio frequency transformer Anodepotential for the tubes I, 3, 5 and 1 is supplied over a commonconductor 53, connected to the positive terminal of the bleeder resistor39. The cathodes of the said tubes are also interfconnected by a commonconductor 55 which extends to an intermediate point 51 on the bleederresistor.

Grid bias for the pentode portion of the first detector tube 3 isprovided by a connection 59 including a grid resistor 6| from thepentode grid thereof to an intermediate point 63 on the bleeder resistormore negative than the point 51 to which the cathodes are connected bythe conductor 55.

Grid bias for the pentode portion of the second intermediate tube 1 isprovided by a connection 6 5 extending from the secondary winding of thel'transformer 49 thereof to a still more negative point 61 on thebleeder resistor.

In order to automatically correlate to incoming signal amplitude thegain in the pentode portions of the radio frequency tube I and the firstintermediate frequency tube 5, the pentode grids thereof are suppliedwith bias over a common conductor 69 connected to an intermediate point1| on the bleeder resistor 39 more negative than the cathode connection51 thereto, the grid connection including a source which suppliesadditional negative potential as the incoming carrier wave increases inamplitude above a pre-determined threshold value determined by thenormal bias supplied from the bleeder resistor. The said sourcecomprises a resistor network 12 constituted by a, plurality of resistors13 and 15 and a potentiometer resistor element 11, the latter beingconnected by a movable contact member 19l to the anode 8| of anautomatic volume control plate-impedance of the automatic volume controltube 83 when no signal is being received.

It is obvious, of course, that the specic network r12 is merelyillustrative of means whereby the space current of the automatic volumecontrol tube may be utilized either for potential application to theconductor 69 or for an additional function later to be explained. Manyother networks for the desired purpose will be obvious to those skilledin the art and no reason is seen for complicating the disclosure bytheir illustration.

The resistance of potentiometer resistor 11 must be high with respect tothe resistance of elements 1 3 and 15. The said automatic volume controltube may be of the screen grid equipotential cathode type having, inaddition to the aforementioned anode, a cathode 85, a control grid 81,and a screen grid 89. The cathode 85 of the automatic volume controltube is connected to a point 9| on the bleeder resistor 39 more negativethan the points thereon to which the cathodes of the amplifier tubes areconnected and to which the control grids in the pentode portions of thefirst detector tube and the second intermediate frequency tube areconnected.

Unidirectional potentials proportional to incoming signal amplitude aresupplied to the control grid and cathode of the automatic volume controltube 83 from the diode output resistor 3| by a plurality of conductors93 and 95, connected, respectively, to the grid of the automatic volumecontrol tube over a resistor 91, and to the negative terminal of thebleeder resistor 39. The resistor Y,91 in the grid connection to theautomatic volume control tube and a condenser 99 connected between thegrid and the cathode thereof provid-e a filter which prevents audiofrequencies from being impressed on the input ter- Preferably, theresistor B4 has a maximum resistance comparable to theA minals of thetube, thus ensuring that the grid potential is unidirectional and isproportional to the average amplitude of the intermediate frequencysignal impressed upon the diode detector 9 from the preceding tubes inthe system.

It will be noted, from the drawing, that the grid of the automaticvolume control tube is connected to the cathode end of the dioderesistor which end becomes mor-e positive as the average rectifiedsignal current increases in amplitude. The negative bias applied to th-esaid grid from the resistor 39 is thus opposed, proportionally to signalamplitude, permitting space current to flow therein, which currenttraverses the resistor network i2 in the output circuit thereof in thedirections indicated by the arrows.

if, therefore, the movable element 19 of the potentiometer is adjustedtoward the end A of the potentiometer resistor 11, to which theconductor 69 connects, the potential developed acrossV the resistor 13is added to the normal bias applied to the pentode grids of the radiofrequency tube i and the first intermediate frequency tub-e 5, with theresult that the gain in the system is always automatically keptinversely proportional to incoming signal amplitude.

In addition to providing the means just ldescribed, whereby the gain inthe system may be kept inversely proportional to the amplitude of anincoming signal without appreciably affecting the selectivity thereof,my invention also includes th-e provision of optionally utilizable meanswhereby the incoming signal may be made use of for automaticallycontrolling the selectivity of the system and for simultaneously givinga measure of gain-control.

As clearly shown in the drawing, the space current path in the triodeportion of the radio frequency amplifier tube I is connected eifectivelyin shunt to the coupling impedance i l, the space current path in thetriode portion of the first detector tube 3 is connected in shunt to theoutput coupling impedance t5, while the space current path in the triodeportion of the first intermediate frequency tube 5 is connectedeffectively in shunt to the primary winding of the output transformer49.

When receiving weak signals, it is desirable that the system shall beselective, and that the gain therein shall be reasonably high, and theconverse is usually true when receiving strong signals. rlhat is to say,when maximum selectivity is required the triode portions of the radiofrequency tube I and the first detector tube 3 must draw substantiallyno current to damp the coupling reactors AI and 65, while the triodeportion of the first intermediate frequency tube 5 must draw sufficientcurrent to effectively loosen the coupling between the windings of thetransformer 49. This condition necessitates a high negative bias on thetriode grids in the tubes I and 3 and zero or slightly negative bias onthe triode grid in the tube 5. Furthermore, if the system is to beplaced in such condition that incoming signals, through the intermediaryof the automatic volume control tube 83, may be utilized to cause theselectivity to decrease in response to increased signal amplitude meansmust be provided for rendering the triode grids in the tubes l and 3less negative and for making the bias on the triode grid in the tube 5sufficiently negative to substantially stop the flow of space currenttherein. The manner in which this is accomplished constitutes animportant phase of my invention and it will now be explained.

Referring again to the drawing, it will be noted that only potentialsincreasing in the negative direction with increase in signal amplitudecan be supplied by the automatic volume control tube 83 with respect tothe common cathode connection 51.

The automatic volume control tube, accordingly, of itself, cannot beutilized to reduce the negative bias normally necessary on the triodegrids of the radio frequency tube I and the detector tube 3, althoughpotentials applied therefrom over a conductor III may be utilized forbiasing the triode grid in the first intermediate frequency tube 5,since they are in the proper direction. The triode grids in the tubes Iand 3, therefore, must be connected to the cathodes thereof over acircuit serially including a separate source of potential whichnormally, or in the weak signal condition, supplies them with highnegative potential and which source may automatically be so controlledby the automatic volume control tube that the potential becomes lessnegative as signal amplitude increases.

To this end, I connect the triode grids in the tubes I and 3 by means ofa common conductor |93, to a point I on the bleeder resistor 39, thepotential of which is the same as that of the cathode connecting point51, or more negative than that of the said point as actually illustratedin the drawing and serially include in the connection a resistor |91across which a variable potential may be. developed by the flow ofcurrent therein from a signal-controlled source. The complete circuit inwhich the current flows includes the secondary winding of apower-transformer H39, the space current path in the triode portion ofthe second intermediate frequency tube 1, the common cathode conductor55 and that portion of the bleeder resistor 39 between the points 51 andIIl5 to which the conductor 55 and the conductor IUS, respectively,extend.

From the triode grid in the second intermediat-e frequency tube, aconnection III extends to the conductor lI whereby negative bias isapplied to the said grid, the bias in value depending upon the amplitudeof an incoming signal and the position of the contact element 19.

For maximum selectivity, therefore, when receiving either weak or strongsignals, the contact element 19 is adjusted toward the end A of theresistor 11, thus permitting the potential of the triode grids in thefirst and second intermediate frequency tubes 5 and 1 to assume a biaspotential determined by the point of connection 1I of the network 12 tothe bleeder resistor 39, which potential is so chosen as to permit spacecurrent to flow in the triode portions of the said tubes. Rectifiedcurrent from the transformer |39 flows in the resistor I01 in thedirection of the arrow and biases the triode grids of the tubes I and 3to the cut-off potential, thus preventing the triode portions of thetubes from drawing plate current and loading the coupling inductors 4Iand 45.

For maximum fidelity, the contact element 19 is adjusted toward the endB of the potentiometer resistor 11. In this position, the potentialsdeveloped across the resistor 15 are applied to the triode grids in thetubes 5 and 1, which potentials, as hereinbefore explained, increasenegatively with increase in amplitude of incoming signals. As the triodegrid of the tube 1 becomes more negative, the flow of rectified currentin the resistor |01 decreases, thus causing the triode grids in thetubes I and 3 to become less negative with the resultthat space currentflows therein and the tuning of the coupling inductors 4I and 45 isbroadened. The more negative potential applied to the triode grid in thetube 5 causes the space current therein to diminish, thus increasing theeffective coupling between the primary and secondary windings of theoutput transformer 49 and increasing the energy transfer thereby.

The increased energy transfer, by properly adjusting the constants ofthe system, may compensate the decrease in energy transfer by thecoupling inductors 4I and 45 or it may be so adjusted that the gain inthe system, as a whole, is lessened as signal amplitude increases.

From a consideration of the foregoing, it will be apparent that, whenthe movable contact element of the potentiometer is adjacent to the endA, the system functions substantially the same as automatic volumecontrol receivers of known types. By adjusting the contact device towardthe point B, however, the system automatically so adjusts itself thatincreased signal strength gives rise to decreased selectivity.

It will also be apparent that, through manipulation of the adjustableresistor 84, current may be caused to flow through the resistor 'I3 o rl5, depending upon the position of the contact element 79, independentlyof the action of the automatic volume control tube. Such being thecase,v

the sensitivity of the receiver can be limited to any desired value,thus rendering the delity of the system independent of the setting ofthe sensitivity control. That is to say, the fidelity of the system willbe determined entirely by the strength of the received signal and thesetting of the contact device 19.

It will, of course, be obvious that, if interference manifests itselfduring the reception of strong signals, by reason of the automaticbroadening of the tuning, the contact element may be moved awayV fromthe point B and toward the point A to an extent sucient to increase theselectivity of the system and to introduce a desired amount of automaticvolume control.

It will be noted from the drawing that I have provided a singleimpedance-coupled amplification stage for the amplification ofintermediate frequency currents and a later transformer--` coupled stagefor the same purpose. It is, however, to be understood that theillustration of the single impedance coupling device 45 is merely forpurposes of explanation, since in receivers constructed by me I haveusually utilized a plurality of such devices and an extra thermionic Itis, however, important to note that the advantages accruing from aplurality of impedancecoupled stages may be obtained through the use ofa single transformer stage in lieu thereof, together with proper circuitconnections whereby the primary and secondary of the transformer areboth loaded in response to the amplitude of an incoming signal. A stageof the type referred to is illustrated in Figure 2 of the drawing.

For the purpose of practicing my invention, as just described, thecoupling transformer 50 comprises a tuned primary winding and a tunedsecondary winding, the primaryrwinding being effectively shunted by theauxiliary space current path in the first tube and the secondary windingbeing effectively shunted by the space current path in the second tube.

The primary and secondary windings of the transformer are so designedand spaced apart that the coupling therebetween is adjustedl for a fairdegree of selectivity. As an alternative, the coupling may be soadjusted that the transformer passes a band of frequencies narrower thanthe band normally passed by the hereinbefore described transformer 49,thus providing the requisite selectivity for the reception of distantstations. Stating the characteristics of the transformer 56 in stillanother way, it may be so designed as to be band-passing when theselectivity of the system is maximum, whereas the transformer 49 is sodesigned that it exhibits band-pass characteristics when the selectivityof the system is minimum.

It will be noted from an inspection of the drawing that the auxiliarygrids 23 in the several tubes are connected to the conductor 103. Suchbeing the case, when the incoming signal increases in amplitude, theseauxiliary grids become less negative, and the several space currentpaths controlled thereby simultaneously exert a shunting action upon theprimary and secondary windings of the transformer. The selectivity ofthe transformer, therefore, de.` creases with increase in signalamplitude while, at the same time, the signal energy transferred fromthe first to the second tube is reduced, thusl simulating the action ofthe impedance device or devices 45 hereinbefore described in detail.

In order to make the advantages accruing from my invention more clearlyapparent, and to point out the reasons why the separate potential sourceis utilized for the purposes hereinbefore de-` scribed, a brief rsum ofmy improved system will now be given.

In the rst place, it is decidedly advantageous to utilize combinedpentode-triode tubes for the functions of amplication and fidelitycontrol, thus obviating the necessity of a plurality of separate tubesfor the latter function as disclosed in my aforementioned priorapplication. also advantageous to utilize a single thermionic device forthe purpose of correlating automatic volume control and automatic delitycontrol to signal strength, the two functions being made available, indifferent degrees, through the manual operation of a single controldevice such as the contact element 19, associated with the potentiometerresistor l'l, the position of which element determines which of the twofunctions is being made available.

Obviously, therefore, one end of the resistor 'l'l must supply automaticvolume control potentials to the control grids of the pentode portionsof the tubes l and 5, while the other end of the resistor must supplyfidelity-control potentials to the tubes I, 3, and 5. Y As hereinbeforeexplained, the potential on the triode grids of the tubes l and 3 iscaused to become less negative with increase in signal strength whilethe potential on the triode grid of tube 5 must become more negative.This necessitates some sort of a potential reversing device,V which issupplied by the triode portion of the tube 1, to the grid of which theend B of the resistor 'l1 is directly connected, and in the outputcircuit of which is included the resistor H11.V

Since one end of the resistor 01 is connected to the triode grids oftubes l and 3 and the other end must be connected to a point in thesystem at the same potential as the common cathode potential of tubes I,3, 5, and 1, or negative with respect thereto, in order to provide thenecessary fixed negative bias on the triode grids of tubes l and 3 whenno current iiows through it, and since the resistor must be included ina cir- It iSA cuit interconnecting the triode anode of the tube 'I and asource of potential, the common source 39 cannot be utilized.

A very important phase of my invention, therefore, consists inthevrealization of the need for an extra potential source such as thetransformer |09, or an equivalent battery (not shown). Naturally,however, in an alternating current powered receiver, the transformer isthe logical source, as illustrated.

If the receiver is of the battery-powered superheterodyne type, atransformer, such as is exemplified by the transformer I I 3, may beutilized instead of the power transformer H09, to supply potential tothe anode of the triode portion of the second intermediate frequencytube l. A coupling condenser might also be used for the same purpose.

Although I have shown and described certain specific embodiments of myinvention, I am fully aware that many modifications thereof arepossible. My invention, therefore, is not to be restricted exceptinsofar as is necessitated by the prior art and by thespi'rit of theappended claims.

I claim as my invention:

1. In a radio receiving system, a thermionic device having more than oneinput electrode, means for supplying to one of the input electrodes ofsaid device unidirectional potentials proportional to the amplitude ofan incoming signal and means for supplying to another input electrode ofthe same device unidirectional potentials inversely proportional to theamplitude of an incoming signal.

2. The invention set forth in claim 1 characterized in that the inputelectrodes are so disposed within the thermionic device as toindependently control an equivalent number of space current paths.

3. The invention set forth in claim 1 characterized in that therst-mentioned unidirectional potential supplying means is constituted bya thermionic device.

4. The invention set forth in claim 1,1characterized in that the secondmentioned unidirectional potential supplying means includes a thermionicdevice.

5. The invention set forth in claim 1 characterized in that thesecond-mentioned unidirectional potential supplying means includes athermionc device having a plurality of input electrodes, a commoncathode, and a plurality of output electrodes.

6. In a radio receiving system, a thermionic device of the type having acathode, output electrode structure, and a plurality of inputelectrodes, the input electrodes being each so disposed with respect tothe cathode and output electrode structure as to be capable ofindependently controlling space current paths individual thereto, anoutput circuit connected to said output electrode structure, means forsupplying incoming signals to one of said input electrodes, means forsupplying to said one electrode unidirectional potentials proportionalto an incoming signal, and means for supplying to another of said inputelectrodes unidirectional potentials inversely proportional to anincoming signal.

7. In a radio receiving system, a thermionic device having a pluralityof input electrodes, a common cathode and output electrode structure,the input electrodes being each so disposed with respect tothe cathodeand output electrode structure as to be capable of independentlycontrolling space current paths individual thereto, thermionic means forsupplying to one of the input electrodes unidirectional potentialsproportional to the amplitude of an incoming signal, thermionic meansfor supplying to another input electrode of the same deviceunidirectional potentials inversely proportional to the amplitude of anincoming signal, a Source of plate potential, a connection from thecathode to a point on said plate potential source, and means forccnductively connecting the anodes of the unidirectional potentialsupplying devices to said plate potential source at a point thepotential of which does not differ greatly from the potential of thepoint to which the cathode of the first mentioned thermionic device isconnected.

8. The invention set forth in claim 'l additionally characterized inthat a source of potential is serially included in the connection fromthe anode of the second-mentioned unidirectional potential supplyingsource to the plate potential source.

9. The invention set forth in claim 7 characterized in that a source ofpotential is included in the connection from the anode of the secondmentioned unidirectional potential source to the plate potential source4and additionally characterized in that the said source of potential isenergized from the said plate potential source.

10. Y In a radio receiving system, a diode detector tube having anoutput circuit, an automatic volume control tube having an input circuitand having an output circuit from which automatic volume controlvpotentials may be derived, a plate potential source, a connection fromthe anode of the detector tube to the negative terminal of said source,a connection from the cathode of the automatic volume control tube to apoint on said source positive with respect to the negativev terminal,and a connection extending from the cathode of the diode detector tubeto the grid of the automatic volume control tube whereby unidirectionalpotentials representative of carrier current amplitude may be impressedupon the grid of the automatic volume control tube and whereby thethreshold value of a signal below which automatic Volume control actiondoes not take place may be varied by varying the connection of thecathode of the automatic volume control tube to the said potentialsource.

11. In a radio receiving system, a plurality of thermionic amplifyingdevices having a common cathode connection, a demodulating device of thediode type having a cathode and an anode and being adapted to cut ofi"when the anode is negative with reference to the cathode, connectionswhereby the cathode of the demodulating device is normally maintainedmore negative than the said common cathode connection, gain controllingmeans for said system including a gain control tube having a. controlgrid connected to the diode cathode, and means for causing the cathodeof the demodulating device to become less negative with respect to saidcommon cathode connection in response to an increase in the amplitude ofa received signal.

12. In a radioreceiving system, an amplifying device having a pluralityof input electrodes, a thermionic signal responsive device having acathode and an anode, an output circuit for said device including aresistor, a plate potential source, a connection from one end of saidresistor to a point on said plate potential source, a connection fromthe cathode of said device to a point on said plate potential source,the potential of the last referred to point with respect to the neg-@tive terminal of said source being substantially the same as thepotential of the rst mentioned point with respect to the said negativeterminal, a connection between the cathode of the amplifying device andthe cathode of the signal-responsive device, a connection between theother end of the resistor and an input electrode of said amplifyingdevice, and means whereby output current from said signal responsivedevice varies inversely with respect to the amplitude of Van incomingcarrier current, whereby said input electrode becomes less negative withrespect to its cathode upon increase in signal amplitude.

13. In a radio receiving system, a thermionic device having two controlelectrodes, a cathode and output electrode structure, a signal outputimpedance device connected to said output structure, said controlelectrodes being so disposed with respect to said cathode and saidoutput structure as to be capable of independent control of anequivalent number of space-currents, means for applying signalpotentials to one of said control electrodes, means for deriving anunidirectional potential varying in response to the amplitude of anincoming carrier wave, and means for applying said potential to theother of said control electrodes in such direction as`to lower theimpedance of its associated space current path upon increase in signalamplitude, whereby automatic control of the damping of the outputimpedance device in response to signal amplitude may be had.

14. In a radio receiving system, a plurality of thermionic devices eachhaving a plurality of input electrodes, a cathode and output electrodestructure, impedance coupling means between said devices, an outputcircuit for one of Ysaid devices including the primary winding of atransformer, the input electrodes in said devices being so disposed withrespect to the cathodes and output electrode structures therein as to becapable of independently controlling an equivalent number of spacecurrent paths, and signal responsive means for causing correspondinginput electrodes in the several devices to be biased in opposite senseproportionately to signal amplitude.

15. In a radio receiving system, a, plurality of thermionic devicestransformer-coupled in cascade, each of said transformers beingconstituted by a pair of coupled tuned windings, means for varying theeffective coupling between the primary and secondary windings of each ofthe transformers, means for controlling the effectiveness of thecoupling-varying means in response to incoming signals, means for makingthe effective coupling between one pair of windings critical when weaksignals are being received, and means for making the coupling betweenthe other pair of windings critical when strong signals are beingreceived.

16. In a, radio receiving system, a plurality of thermionic devicestransformer-connected in cascade, the coupling between the primary andsecondary windings of one of said transformers being such and theconstants of the said windings being,r so chosen that the transformerpasses a narrow band of frequencies, the coupling between the primaryand secondary windings of another of said transformers being such thatthe transformer passes a wide band of frequencies, means for imposing aload on the primary and secondary windings of the first-mentionedtransformer, means for providing a path effectively in shunt to theprimary winding of the second-mentioned transformer, and means forincreasing the load on the windings of the iirst transformer in responseto an increase in signal amplitude and for increasing the impedance ofthe shunting path associated with the primary winding of the secondmentioned transformer in response to an increase in signal amplitude.

17. In a radio receiving system at least three transformer-coupledthermionic devices, each of the transformers comprising a pair ofcoupled tuned circuits, signal responsive means for increasing theeffective coupling between one pair of coupled tuned circuits inresponse to an increase in the amplitude of a received signal, andsignal responsive means for decreasing the effective coupling betweenthe other pair of coupled tuned circuits in response to an increase inthe amplitude of a received signal.

18. The invention set forth in claim 16 characterized in that each ofthe signal responsive means is constituted by a thermionic device.

19. In a radio receiver, means for changing the selectivity of saidreceiver in response to a change in the amplitude of an incoming signal,means for changing the sensitivity of said receiver in response to achange in the amplitude of an incoming signal, and means for manuallyadjusting the sensitivity of the receiver without affecting said firstmeans.

GEORGE L. BEERS.

